Halftone printers emit a combination of colorants to form both single color and multicolor printed images using a comparatively small number of primary colors. Inkjet printers and three-dimensional object inkjet printers eject patterns of ink drops to form both single and multicolor printed images. In a three-dimensional object inkjet printer, one or more printheads eject drops of ink onto a surface of an object that is formed from a build material. In many embodiments, the printer forms the printed image in a multi-pass printing process where the inkjets in the printheads apply multiple layers of ink to the form the printed image on the surface of the object. The printer applies multiple layers of ink using a single halftone pattern of ink drops to build up a sufficient amount of ink for the printed image to be clearly visible on the surface of the object. Combinations of multiple ink colors, such as cyan, magenta, yellow, and black (CMYK) inks, form images with a wide range of perceptible colors on the surface of the object.
Many modern inkjet printers receive digital data corresponding to a printed image. The digital data for a printed image often include data corresponding to printed colors that are encoded in a continuous tone (contone) format. In a contone format, a single two-dimensional location in the image, which is referred to as a contone pixel, can have a wide range of colors that are formed from different intensity levels of basic colors such as red, green, blue (RGB) colors. In other embodiments, the colors of contone pixels are encoded in a device independent color space such as the L*a*b* color space or other color spaces known to the art. Halftone printers, however, produce images from patterns of colorants using the limited number of ink or toner colors in a halftone output that includes a pattern of the colorant, such as a pattern of ink drops or toner. For example, the color of a single pixel of contone image data may be reproduced using a pattern of multiple separate ink drops that reproduce the contone color on a physical print medium. The physical properties of the ink drops affect the perceptible intensity of printed ink colors since the ink drops spread on the image receiving surface and generally have circular shapes instead of the square shapes associated with contone pixels.
Three-dimensional inkjet printers employ tone reproduction curves (TRCs) to convert image data that are provided in the contone formats to values such that when the values are halftoned, the printed ink patterns provide an accurate reproductions of the original contone images. For example, a set of gray contone pixels with a relative level of 50% between pure-black (100% level) and pure-white (0% level) generates a halftone pattern with half of the pixels being assigned a black ink drop and the other half of the pixels being left blank, which corresponds to a background material under the printed image. In some instances the background material has a white or light gray color to provide a neutral background for a printed image. Because the ink drops spread on the surface of the object, however, printing the direct halftone pattern produces a printed image that is darker than the intended 50% level in one printer configuration. A digital controller in the printer uses the TRC to generate modified contone image data that includes adjustments for the intensity levels of some or all of the contone pixels in the original digital image data. For example, in one configuration a TRC reduces a contone pixel levels from 50% to 40%. The printer then uses halftone processes that are known to the art to convert the modified contone image data into patterns of image data corresponding to printed colorant patterns in a printed image.
In modern printing systems, the TRCs are typically embodied as digital data structures that have a predetermined number of potential input and output values based on the number of discrete levels that can be represented using digital data. For example, an 8-bit TRC generates a single component value in a range of 0-255 (28). Since the number of perceptible color levels in a printed image is often much greater than the number of discrete levels represented in a TRC, modern printers often employ complex techniques such as spatial adjustments to halftone pixels to produce an apparent increase in the color resolution of printed images. In a spatial adjustment process, a digital processor in a printer introduces randomized errors into halftoned image data based on the location of pixels in a two-dimensional image to increase the apparent number of color levels in the image. Of course, another solution is to increase the number of digital bits used in the TRCs (e.g. 12 or 16 bits) to increase color resolution, but such changes require both extensive hardware and software modifications to printers and image processing software to produce an entire image process pipeline with the increase resolution, which is often impractical. Consequently, improved systems and methods for controlling halftone printing, including halftone printed images in three-dimensional object printers to increase the effective color resolution of printed images would be beneficial.